1. Field of the Invention
The present invention relates to a circuit for controlling RF PAs (Radio Frequency Power Amplifiers), and more specifically, to an RF PA controller circuit that operates with a polar transmitter.
2. Description of the Related Art
RF (Radio Frequency) transmitters and RF power amplifiers are widely used in portable electronic devices such as cellular phones, laptop computers, and other electronic devices. RF transmitters and RF power amplifiers are used in these devices to amplify and transmit the RF signals remotely. RF PAs are one of the most significant sources of power consumption in these electronic devices, and their efficiency has a significant impact on the battery life on these portable electronic devices. For example, cellular telephone makers make great efforts to increase the efficiency of the RF PA circuits, because the efficiency of the RF PAs is one of the most critical factors determining the battery life of the cellular telephone and its talk time.
FIG. 1 illustrates an example of a conventional RF transmitter circuit, including a polar modulator 102 (sometimes referred to as a polar transmitter) and an external power amplifier (PA) 104. For example, the RF transmitter circuit may be included in a cellular telephone device using one or more cellular telephone standards (modulation techniques) such as EDGE, UMTS (Universal Mobile Telephony System) or CDMA (Code Division Multiple Access), although the RF transmitter circuit may be included in any other type of RF electronic device. For purposes of illustration only, the RF transmitter circuit will be described herein as a part of a cellular telephone device. The polar modulator 102 includes an envelope generator 304 for generating an amplitude component 105 of the desired RF modulated signal and a phase generator 305 for generating the phase component 307 of the desired RF modulated signal. A phase modulator 306 receives the phase component 307 and modulates the signal onto an RF carrier for output to a variable gain amplifier (VGA) 107 as the phase modulated signal 108. The amplitude component 105 modulates the gain of the VGA 107, thus combining the amplitude component 105 and phase-modulated component 108 to generate the RF signal 106 to be amplified by the PA 104 and transmitted 110 remotely by an antenna (not shown). For example, the RF signal 106 may be an RF signal modulated by the polar modulator 102 according to the EDGE, UMTS or CDMA standard.
The RF power amplifier 104 in general includes an output transistor (not shown) for its last amplification stage. When an RF modulated signal 106 is amplified by the RF PA 104, the output transistor tends to distort the RF modulated signal 106, resulting in a wider spectral occupancy at the output signal 110 than at the input signal 106. Since the RF spectrum is shared amongst users of the cellular telephone, a wide spectral occupancy is undesirable. Therefore, cellular telephone standards typically regulate the amount of acceptable distortion, thereby requiring that the output transistor fulfill high linearity requirements. In this regard, when the RF input signal 106 is amplitude-modulated, the output transistor of the PA 104 needs to be biased in such a way that it remains linear at the peak power transmitted. This typically results in power being wasted during the off-peak of the amplitude of the RF input signal 106, as the biasing remains fixed for the acceptable distortion at the peak power level.
Certain RF modulation techniques have evolved to require even more spectral efficiency, and thereby forcing the RF PA 104 to sacrifice more efficiency. For instance, while the efficiency at peak power of an output transistor of the PA 104 can be above 60%, when a modulation format such as WCDMA is used, with certain types of coding, the efficiency of the RF PA 104 falls to below 30%. This change in performance is due to the fact that the RF transistor(s) in the RF PA 104 is maintained at an almost fixed bias during the off-peak of the amplitude of the RF input signal 106.
Certain conventional techniques exist to provide efficiency gains in the RF PA 104. One conventional technique is Large Signal Polar, shown in FIG. 2. The Large Signal Polar technique is a variation of the polar modulator described in FIG. 1. Here the amplitude component 105 and the phase modulated signal 108 of the desired RF modulated signal are applied separately to 2 ports of the power amplifier 104, i.e., its supply voltage port (Vcc) 109 and its RF input port 107, respectively. Modulating the supply voltage of the PA 104 dynamically adjusts the bias during the amplitude variations of the RF input signal 106 and therefore, in theory, improves the efficiency of PA 104. However, the Large Signal Polar technique often fails to provide significant net efficiency gains, because the supply voltage 109 to the PA 104 cannot be varied in an energy-efficient way to accommodate the large variations in the amplitude signal of the desired RF modulated signal and thus it fails to provide a substantial energy efficiency gain while maintaining the required linear amplification of the RF signal in the RF PA 104. This is mainly due to the difficulty in realizing a fast, accurate, wide range, and energy efficient voltage converter to drive the supply voltage of the RF PA 104.
The conventional Large Signal Polar technique can function better only if a variable power supply with a very large variation range is used to adjust the supply voltage based on the amplitude component 105 of the desired RF modulated signal, while not reducing the efficiency of the RF transmitter by power consumed by the power supply itself. However, the variable power supply, which is typically comprised of a linear regulator 111 that varies its output voltage on a fixed current load such as the PA 104 in linear mode, by principle reduces the supply voltage 109 at constant current and by itself consumes the power resulting from its current multiplied by the voltage drop across the linear regulator 111 when there is a large drop in the amplitude signal 105 of the desired RF modulated signal. This results in little change in the overall battery power being consumed by the RF transmitter, because any efficiency gained in the RF PA 104 is mostly lost in the linear regulator 111 itself.
Quite often, the conventional methods of controlling a PA fail to address the amplitude-to-phase re-modulation (AM-to-PM) which occurs in a non-frequency linear device such as a PA. Thus, the conventional methods are not suitable for the common types of PAs for use in common mobile telephony or mobile data systems because the required spectral occupancy performance is compromised by the AM to PM distortion.
Thus, there is a need for an RF PA system that is efficient over a wide variety of modulation techniques and results in a significant net decrease in power consumption by the RF PA circuit. There is also a need for a PA controller that can correct the AM to PM effects.